The present invention relates generally to optics, fiber optics, and optical networks. More particularly, the present invention relates to the control of optical gain profiles through the use of a dynamic gain tilt and curvature control device that has applications in optical networks, optical communications, and optical instrumentation.
Optical fibers are replacing copper cables at a rapid pace as the transition medium for communication systems. Optical fibers are used in the long-haul telecommunication backbone, as well as in regional and metropolitan systems to service the fast growing need of wider bandwidth and faster speed fueled by Internet usage. A dramatic increase in the information capacity of an optical fiber can be achieved by the simultaneous transmission of optical signals over the same fiber from many different light sources having properly spaced peak emission wavelengths. By operating each source at a different peak wavelength, the integrity of the independent messages from each source is maintained for subsequent conversion to electric signals at the receiving end. This is the basis of wavelength division multiplexing (WDM). To ensure smooth and efficient flow of information, optical networks should have intelligence built in. Dynamically controllable devices are one of the key building blocks for smart optical networks.
For optical signals to travel long spans in optical networks based on WDM or dense WDM (DWDM) without expensive optical-to-electrical-to-optical (OEO) conversion, optical amplifiers are used. Today, the most often used optical amplifiers are Erbium Doped Fiber Amplifiers (EDFAs). Optical networks need to have uniform power levels across the channels to minimize detection noise and signal saturation problems. However, in practice, the widely used EDFAs have non-linear gain profiles. For static EDFA gain, the gain profile can be compensated by passive gain flattening filters (GFF) based on thin film dielectric filtering or Bragg grating technologies. However, in addition to this static problem, there are several factors that cause dynamic wavelength dependent gains in the optical networks. These factors include (a) saturation effect of the amplifier medium; (b) pump laser power and different gain settings; and (c) number of channels (changes due to adding and dropping channels) input powers.
FIG. 1a shows the measured gain profiles of an EDFA (with a GFF) set at different gains. It is evident that the gain profile tilts significantly ( greater than 10 dB) over the 1528 nm to 1563 nm wavelength band. In addition, there is a noticeable change in slope below approximately 1540 nm when the gain setting is changed from 20 to 10 dB. The GFF in this case has been optimized for 23 dB amplification. As a rule, if the gain is set below the GFF-optimized gain, the tilt is positive; if the gain is set above the GFF-optimized gain, the tilt is negative. This is illustrated in FIG. 1b, which shows the tilt for an EDFA that has been gain-flattened with a GFF at 20 dB.
To compensate dynamically for this wavelength dependent gain, dynamic gain equalizing (DGE) devices have to be used. Several DGE solutions have been proposed. They fall into two general categories: dynamic channel equalizer (DCE) and dynamic spectral equalizer (DSE). Dynamic channel equalizers use a grating or thin film filters to de-multiplex (demux), control, and then multiplex (mux) individual channels to achieve equalization. Although they offer good flexibility down to individual channel levels, they are sensitive to channel number and spacing, and thus are not scalable. They are also complicated in design, large in package size, and are very expensive ( greater than $1000/channel).
Dynamic spectral equalizers, on the other hand, only control the overall spectral shape without the demux/mux steps and offer important scalability. They can be used for different DWDM systems with different channel numbers and spacing. The current solutions are based on multiple stage systems, where each stage controls a portion of the spectrum. Their disadvantages include high cost, high insertion loss, and unreliability. They are still complicated in design, packaging (e.g. alignment of many stages), and control. Therefore, they are still expensive ( greater than $5000/channel).
A known optical control technique is tunable optical retardation. Tunable optical retardation can be implemented in a number of ways. For example, a liquid crystal can be used to implement this function. Liquid crystals are fluids that derive their anisotropic physical properties from the long-range orientational order of their constituent molecules. Liquid crystals exhibit birefringence and the optic axis can be reoriented by an electric field. This switchable birefringence is the mechanism underlying nearly all applications of liquid crystals to optical devices.
A liquid crystal variable wave plate is illustrated in FIG. 2a. A layer of nematic liquid crystal 1 is sandwiched between two transparent substrates 2 and 3. Transparent conducting electrodes 4 and 5 are coated on the inside surfaces of the substrates. The electrodes are connected to a voltage source 6 through an electrical switch 7. Directly adjacent to the liquid crystal surfaces are two alignment layers 8 and 9 (e.g., rubbed polyimide) that provide the surface anchoring required to orient the liquid crystal. The alignment is such that the optic axis of the liquid crystal is substantially the same through the liquid crystal and lies in the plane of the liquid crystal layer when the switch 7 is open.
FIG. 2b depicts schematically the liquid crystal configuration in this case. The optic axis in the liquid crystal 1 is substantially the same everywhere throughout the liquid crystal layer. FIG. 2c shows the variation in optic axis orientation 12 that occurs because of molecular reorientation when the switch 7 is closed.
As an example, we consider a switchable half wave retardation plate. For this case, the liquid crystal layer thickness, d, and birefringence, xcex94n, are chosen so that                                           Δ            ⁢                          xe2x80x83                        ⁢            nd                    λ                =                  1          2                                    (        1        )            
where xcex is the wavelength of the incident light. In this situation, if linearly polarized light with wave vector 13 is incident normal to the liquid crystal layer with its polarization 14 making an angle 15 of 45 degrees with the plane of the optic axis of the liquid crystal, the linearly polarized light will exit the liquid crystal with its polarization direction 18 rotated by 90 degrees from the incident polarization.
Referring now to FIG. 2c, the optic axis in the liquid crystal is reoriented by a sufficiently high field. If the local optic axis in the liquid crystal makes an angle xcex8 with the wave vector k of the light, the effective birefringence at that point is                               Δ          ⁢                      xe2x80x83                    ⁢                      n            eff                          =                                                            n                e                            ⁢                              n                o                                                                                                          n                    o                    2                                    ⁢                                      cos                    2                                    ⁢                                      xe2x80x83                                    ⁢                  Θ                                +                                                      n                    e                    2                                    ⁢                                      sin                    2                                    ⁢                                      xe2x80x83                                    ⁢                  Θ                                                              -                      n            o                                              (        2        )            
where no and ne are the ordinary and extraordinary indices of the liquid crystal, respectively. The optic axis in the central region of the liquid crystal layer is nearly along the propagation direction 13. In this case, according to Eq. 2, both the extraordinary 16 and ordinary components 17 of the polarization see nearly the same index of refraction. Ideally, if everywhere in the liquid crystal layer the optic axis were parallel to the direction of propagation, the medium would appear isotropic and the polarization of the exiting light would be the same as the incident light.
Before leaving a discussion of this tunable half wave plate, it is useful for later understanding of the current invention to give a geometrical representation of the polarization as afforded by the Poincare sphere. FIG. 3 shows a projection of the Poincare sphere as viewed from the top. In this view, circular polarization 21 is at the center of the projection; all states of linear polarization occur on the equatorxe2x80x94the outer most circle. Two diametrically opposed points on the sphere correspond to orthogonal polarizations. For example, the two points 22 and 23 represent orthogonal linear polarizations, as do points 24 and 25. When light propagates through a liquid crystal layer, or any other birefringent medium, its polarization will change continuously; this change can be mapped as a continuous curve on the sphere. The curve 26 shown on the sphere in FIG. 3 represents the changes in polarization that are experienced for the situation of FIG. 2b. Point 22 corresponds to the incident polarization and point 23 to the exit polarization of the unactivated liquid crystal cell. Observe that they are orthogonal.
In view of the foregoing, it would be highly desirable to provide a single-stage solution for dynamic (or tunable) gain tilt compensation. Ideally, such a solution would utilize tunable optical retardation, simplified control techniques, and could be implemented in a very compact package. In addition, such a solution would ideally provide a mechanism for closely fitting a non-linear spectral profile.
An apparatus for processing an optical beam has at least one variable optical element to dynamically alter the polarization state of a polarized optical beam to form a polarization-altered optical beam. The polarization-altered optical beam includes elliptical polarization. The at least one variable optical element is a compound birefringent crystal with a designed retardation response to temperature variations. In one embodiment, the compound birefringent crystal has a designed retardation response that is substantially invariant with operating temperature variations. At least one wave plate processes the polarized optical beam. Each wave plate has a selected retardation, order of retardation, and orientation. A polarization analyzer is operative in conjunction with the at least one variable optical element and wave plate to alter the transmitted amplitude of the polarization-altered optical beam as a function of wavelength, and thereby produce an output optical beam with transmitted amplitude adjusted as a function of wavelength.